lib: move strncpy_from_unsafe() into mm/maccess.c

To fix build errors:
kernel/built-in.o: In function `bpf_trace_printk':
bpf_trace.c:(.text+0x11a254): undefined reference to `strncpy_from_unsafe'
kernel/built-in.o: In function `fetch_memory_string':
trace_kprobe.c:(.text+0x11acf8): undefined reference to `strncpy_from_unsafe'

move strncpy_from_unsafe() next to probe_kernel_read/write()
which use the same memory access style.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Fixes: 1a6877b9 ("lib: introduce strncpy_from_unsafe()")
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>